1. Field of the Invention
The present invention relates to a method for surface treatment to form a layer of a nitride or carbonitride of at least one element selected from chromium (Cr), vanadium (V), and titanium (Ti) on the surface of iron or iron alloy articles, such as dies, jigs, and machine parts.
2. Description of the Prior Art
It is known that iron and iron alloy articles (referred to as articles to be treated hereinafter) are improved in abrasion resistance, seizure resistance, oxidation resistance, corrosion resistance, etc., when they are coated with a surface layer of a carbide, nitride, or carbonitride of one or more than one element of the group chromium, vanadium, and titanium. There have recently been proposed several methods for forming the surface coating layer. According to one of them, an article to be treated is coated with a surface layer of a nitride or carbonitride of chromium, vanadium, or titanium by the aid of plasma chemical vapor deposition from a halide of chromium, vanadium, or titanium. (For example, Japanese Laid-open Patent Publication Nos. 65357/1980 and 164072/1980.) An advantage of this method is that it is possible to form the surface layer without causing heat-induced strain to the base metal of the article to be treated because the treatment is carried out at a temperature lower than the Ac.sub.1 transformation point of iron, which is about 650.degree. C. However, this method has a disadvantage that it is difficult to form a surface layer superior in adhesion and uniformity of the thickness of the layer. An additional disadvantage is that the treatment needs complex steps and an expensive apparatus, and the treatment has to be carried out in hydrogen or under reduced pressure.
In order to overcome the disadvantages of the conventional method, the present inventors previously completed an invention relating to a method for surface treatment which comprises forming a surface layer of a nitride or carbonitride of chromium, vanadium, or titanium on the surface of an article to be treated at a low temperature below 700.degree. C. (U.S. patent application No. 23,862) According to this method, at first, an article to be treated is subjected to nitriding to form a layer of iron-nitrogen compound or iron-carbon-nitrogen compound on the surface of an article to be treated. Subsequently, the article is heated at a temperature below 700.degree. C. in a treating agent composed of a refractory powder, such as alumina, a material containing chromium, vanadium, or titanium, and a halogenated ammonium salt and/or a metal halide, said treating agent being fluidized by argon or the like. The second treatment permits chromium, vanadium, or titanium to diffuse into the compound layer formed by the nitriding. In this way, an article to be treated is formed with a surface layer of a nitride or carbonitride of chromium, vanadium, or titanium. (This method is referred to as dual treatment method hereinafter.)
The disadvantage of the dual treatment method is that it is necessary to carry out treatment twice at almost the same temperature, the first for forming a nitride or carbonitride of iron, and the second for diffusing chromium, vanadium, or titanium, thereby forming a nitride or carbonitride of one of these elements. Therefore, it is poor in efficiency and consumes a large amount of energy. In order to eliminate this disadvantage, the present inventors carried out extensive studies and found a method for forming in a single step the same surface layer of nitride or carbonitride as that formed by the dual treatment method. It is important to note that in the dual treatment method, the nitride or carbonitride constituting the surface layer can be made from any of vanadium (V), chromium (Cr), titanium (Ti), tungsten (W), and molybdenum (Mo) which have a large negative free energy for the formation of nitrides or carbonitrides, whereas in the single-step method of the present invention, the nitride or carbonitride can be made from vanadium, chromium, and titanium, but cannot be made from tungsten and molybdenum despite many attempts. In other words, the reaction associated with the formation of the surface layer in the present invention is not elucidated by the magnitude of free energy. Therefore, the present invention is not easily conceived on the basis of the prior art.